TireSizesFinder

305/40R20 vs 265/40R21

265/40R21 is 0.26 in (6.6 mm) shorter than 305/40R20, a −0.88% diameter difference. With the speedometer set for 305/40R20, an indicated 60 mph is 59.5 mph on 265/40R21.

Is this a safe swap?

Diameter difference −0.88%: under 1%

By overall diameter alone, 265/40R21 is a near match for 305/40R20; the speedometer changes by less than 1%. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 305/40R20 (29.61 in) vs 265/40R21 (29.35 in)305/40R2029.61 in × 12.01 in265/40R2129.35 in × 10.43 in
Side profile and tread width, drawn to the same scale.
305/40R20 vs 265/40R21 differences
Measurement305/40R20265/40R21Difference% diff
Overall diameter29.61 in (752.0 mm)29.35 in (745.4 mm)−0.26 in (−6.6 mm)−0.88%
Sidewall height4.80 in (122.0 mm)4.17 in (106.0 mm)−0.63 in (−16.0 mm)−13.11%
Section width12.01 in (305.0 mm)10.43 in (265.0 mm)−1.57 in (−40.0 mm)−13.11%
Circumference93.01 in (2,362.5 mm)92.19 in (2,341.7 mm)−0.82 in (−20.7 mm)−0.88%
Revolutions per mile681687+6+0.89%
True speed with the speedometer calibrated for 305/40R20
IndicatedActual on 305/40R20Actual on 265/40R21Difference
30 mph30.0 mph29.7 mph−0.3 mph
60 mph60.0 mph59.5 mph−0.5 mph
75 mph75.0 mph74.3 mph−0.7 mph

Ride-height change: 265/40R21 lowers the vehicle by 0.13 in (3.3 mm) at the axle (half the −6.6 mm diameter difference).

265/40R21 is 0.26 in shorter than 305/40R20 and 40 mm narrower, a 0.88% difference in overall diameter.

With the speedometer calibrated for 305/40R20, an indicated 60 mph is actually 59.5 mph on 265/40R21, and revolutions per mile change from 681 to 687.

265/40R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 305/40R20.

Size pages

Other comparisons with 305/40R20

Other comparisons with 265/40R21

Source: Calculated from standard tire geometry formulas. Verified . How these numbers are calculated.